Avoid compiler warnings from function pointer conversions

gcc8 -Wextra prints a warning when reinterpret_cast is used for conversion
between different types of function pointers. Avoid that by adding
Glib::bitwise_equivalent_cast<>() with a union with members of
the two types of function pointers.

* glib/src/optiongroup.ccg: Use Glib::bitwise_equivalent_cast<>() to convert
from a function pointer to void*.

See https://github.com/libsigcplusplus/libsigcplusplus/issues/1

5 files changed, 29 insertions, 13 deletions

diff --git a/gio/giomm/socketsource.cc b/gio/giomm/socketsource.cc
index af1e9740..934ca004 100644
--- a/gio/giomm/socketsource.cc
+++ b/gio/giomm/socketsource.cc
@@ -73,7 +73,8 @@ SignalSocket::connect(const sigc::slot<bool, Glib::IOCondition>& slot,
   GSource* const source =
     g_socket_create_source(socket->gobj(), (GIOCondition)condition, Glib::unwrap(cancellable));
   return Glib::Source::attach_signal_source(
-    slot, priority, source, context_, (GSourceFunc)&giomm_signalsocket_callback);
+    slot, priority, source, context_,
+    Glib::bitwise_equivalent_cast<GSourceFunc>(&giomm_signalsocket_callback));
 }
 
 SignalSocket
@@ -96,7 +97,7 @@ SocketSource::SocketSource(const Glib::RefPtr<Socket>& socket, Glib::IOCondition
   const Glib::RefPtr<Cancellable>& cancellable)
 : IOSource(
     g_socket_create_source(socket->gobj(), (GIOCondition)condition, Glib::unwrap(cancellable)),
-    (GSourceFunc)&giomm_socketsource_callback)
+    Glib::bitwise_equivalent_cast<GSourceFunc>(&giomm_socketsource_callback))
 {
 }
 
diff --git a/glib/glibmm/main.cc b/glib/glibmm/main.cc
index 30a294c4..ff609a43 100644
--- a/glib/glibmm/main.cc
+++ b/glib/glibmm/main.cc
@@ -27,6 +27,7 @@
 #include <glibmm/exceptionhandler.h>
 #include <glibmm/wrap.h>
 #include <glibmm/iochannel.h>
+#include <glibmm/utility.h>
 #include <algorithm>
 #include <map> // Needed until the next ABI break.
 
@@ -540,8 +541,8 @@ SignalChildWatch::connect(const sigc::slot<void, GPid, int>& slot, GPid pid, int
   if (priority != G_PRIORITY_DEFAULT)
     g_source_set_priority(source, priority);
 
-  g_source_set_callback(source, (GSourceFunc)&glibmm_child_watch_callback, conn_node,
-    &SourceConnectionNode::destroy_notify_callback);
+  g_source_set_callback(source, Glib::bitwise_equivalent_cast<GSourceFunc>(&glibmm_child_watch_callback),
+    conn_node, &SourceConnectionNode::destroy_notify_callback);
 
   conn_node->install(source);
   g_source_attach(source, context_);
@@ -1309,7 +1310,7 @@ IOSource::IOSource(PollFD::fd_t fd, IOCondition condition) : poll_fd_(fd, condit
 
 IOSource::IOSource(const Glib::RefPtr<IOChannel>& channel, IOCondition condition)
 : Source(g_io_create_watch(channel->gobj(), (GIOCondition)condition),
-    (GSourceFunc)&glibmm_iosource_callback)
+    Glib::bitwise_equivalent_cast<GSourceFunc>(&glibmm_iosource_callback))
 {
 }
 
diff --git a/glib/glibmm/utility.h b/glib/glibmm/utility.h
index eaf4c15a..39c74a9f 100644
--- a/glib/glibmm/utility.h
+++ b/glib/glibmm/utility.h
@@ -158,6 +158,25 @@ destroy_notify_delete(void* data)
   delete static_cast<T*>(data);
 }
 
+// Conversion between different types of function pointers with
+// reinterpret_cast can make gcc8 print a warning.
+// https://github.com/libsigcplusplus/libsigcplusplus/issues/1
+/** Returns the supplied bit pattern, interpreted as another type.
+ *
+ * When reinterpret_cast causes a compiler warning or error, this function
+ * may work. Intended mainly for conversion between different types of pointers.
+ */
+template <typename T_out, typename T_in>
+inline T_out bitwise_equivalent_cast(T_in in)
+{
+  union {
+    T_in in;
+    T_out out;
+  } u;
+  u.in = in;
+  return u.out;
+}
+
 } // namespace Glib
 
 #endif /* DOXYGEN_SHOULD_SKIP_THIS */
diff --git a/glib/src/optiongroup.ccg b/glib/src/optiongroup.ccg
index 95177ce4..483e8354 100644
--- a/glib/src/optiongroup.ccg
+++ b/glib/src/optiongroup.ccg
@@ -571,12 +571,7 @@ OptionGroup::CppOptionEntry::allocate_c_arg()
   case G_OPTION_ARG_CALLBACK:
   {
     // The C arg pointer is a function pointer, cast to void*.
-    union {
-      void* dp;
-      GOptionArgFunc fp;
-    } u;
-    u.fp = &OptionGroup::option_arg_callback;
-    carg_ = u.dp;
+    carg_ = Glib::bitwise_equivalent_cast<void*>(&OptionGroup::option_arg_callback);
 
     // With all compiler warnings turned on and a high optimization level
     // it's difficult to cast a function pointer to a void*. See bug 589197.
diff --git a/tests/glibmm_vector/main.cc b/tests/glibmm_vector/main.cc
index 31138cf0..dd83553e 100644
--- a/tests/glibmm_vector/main.cc
+++ b/tests/glibmm_vector/main.cc
@@ -198,12 +198,12 @@ public:
   {
     if (glist_)
     {
-      g_list_foreach(glist_, reinterpret_cast<GFunc>(g_object_unref), nullptr);
+      g_list_foreach(glist_, Glib::bitwise_equivalent_cast<GFunc>(g_object_unref), nullptr);
       g_list_free(glist_);
     }
     if (gslist_)
     {
-      g_slist_foreach(gslist_, reinterpret_cast<GFunc>(g_object_unref), nullptr);
+      g_slist_foreach(gslist_, Glib::bitwise_equivalent_cast<GFunc>(g_object_unref), nullptr);
       g_slist_free(gslist_);
     }
     if (garray_)